Metal tubes such as brake tubes and metal fuel lines are required to have chemical resistance, salt spray corrosion resistance, impact resistance as well as heat resistance. Protective coatings are generally applied to the metal which are capable of withstanding chemical erosion and mechanical erosion.
Fluorocarbon-based resins are known to provide excellent protection to metals, as well as imparting a durable and aesthetic appearance. Unfortunately, the resins themselves adhere very poorly to metal substrates.
One method for increasing adhesion of a fluorocarbon-based coating to a metal is to apply a primer coating or other surface treatment, which improves the adhesion of the topcoat to the substrate and also improves the corrosion resistance of the substrate. U.S. Pat. No. 6,500,565 and US 2002/0090528 disclose a multilayer corrosion resistant PVDF coating for automobile brake tubes. The metal substrate is surface treated with a zinc or zinc-nickel plating layer and a chromate film of trivalent chromium, and was primed with an epoxy layer. U.S. Pat. No. 4,379,885 discloses an acrylic based coating composition especially useful as a primer for metal substrates.
The problem with these methods for providing a PVDF coating is that they involve two steps to ensure good adhesion of the PVDF film—an undercoating followed by the PVDF coating, which adds the expense and complexity of an added manufacturing step.
A one-step polyvinylidene coating for metal surfaces is described in U.S. Pat. No. 5,130,201. A very wide range (10 to 95%) of vinylidene fluoride and a very wide range (1 to 80%) of a poorly compatible resin, such as an epoxy resin, is described. A maximum of 70% PVDF resin and a 7.5% minimum of a curing agent is exemplified.
The problem with the one-step coating of the '201 reference is that it sacrifices some of the protective properties of the coating by teaching lower levels of PVDF in the coating, and it also describes a highly cross-linked system with little flexibility, that can result in cracking of the coating.
There is a need for a one-step coating having good adhesion, excellent chemical resistance such as brake fluid resistance, excellent protective properties, and flexibility for coating metal substrates, especially in applications such as for metal fuel lines and brake tubes.
Surprisingly it has been found that a fluoropolymer coating having good adhesion, superior chemical resistance, impact resistance, corrosion resistance, abrasion resistance, heat resistance and good flexibility, can be formed from a composition having greater than 75% fluoropolymer, 5-20% of an acrylic resin, 1-15% of a polyepoxide resin, and low level of crosslinking. This coating provides the properties required for metal fuel lines and brake tubes. An additional advantage is that brake tubes coated with the composition of the invention have excellent brake fluid resistance.